Abstract
As a new tunnel presupport construction technology, the freeze-sealing pipe roof method (FSPR) has been successfully applied to the Gongbei Tunnel Project for the first time in China. To overcome the practical difficulties of this new method in the freezing construction process, such as welding difficulties of the profiled freezing tube, refrigerant leakage, and long construction period, based on the principle of an equal cooling capacity, an improved design of freezing tube was proposed. By designing three different pipe configurations and using scaled model tests and numerical simulation, the feasibility of the improvement and the difference in the freezing temperature field were studied. The research results show that the improved design with double circular freezing tubes exhibited a lower temperature and a higher cooling rate during the test, and the time required to meet the freezing design requirements was significantly shortened. Finally, a composite structure of “pipe roof and frozen soil curtain” with a better carrying capacity and water sealing performance was formed. It was recommended to strengthen the temperature monitoring at the pipe wall in the preliminary stage of the freezing construction to ensure the water sealing effect between the pipes. In the later stage, attention should be paid to monitoring the thickness of the frozen curtain and reducing excessive frost heave through technical means such as cooling capacity control. The improvement and configurations proposed in this research could effectively replace profiled freezing tubes in the original project, better adapt to horizontal freezing projects of similar curved tunnels, achieve the freezing goal faster, and provide a reference for the promotion of the FSPR.
Highlights
As a new tunnel presupport construction technology, the freeze-sealing pipe roof method (FSPR) has been successfully applied to the Gongbei Tunnel Project for the first time in China
The freeze-sealing pipe roof method (FSPR) which combines the advantages of the pipe roof method and the artificially ground freezing method has been proposed. e principle of this new method is that, firstly, several sections of large-diameter and closely arranged steel pipes were jacked into the stratum to form a pipe roof, and the artificially ground freezing was carried out by installing freezing tubes inside the pipe wall to form a certain thickness of frozen soil curtain around the pipe roof, forming a supporting system with both bearing capacity and water sealing performance
As the freezing tubes were near the horizontal direction of the pipes, the temperature in the horizontal direction was lower than that in the vertical direction, and the shape of the isotherm at a far distance was approximately elliptical. e average thicknesses of the frozen wall formed between the two pipes in Configurations A and B were 252 mm and 281 mm, respectively, which means that a reliable water sealing structure between the pipes was formed
Summary
Some of the concrete pipes and the hollow pipes were taken for the horizontal arrangement to design three different pipe configurations (Figure 5). (3) Configuration C: e combination of two concrete pipes and three hollow pipes (including improved double circular freezing tubes). In Configurations A and B, temperature measurement points were arranged on the midline between the two pipes and in the vertical and horizontal directions of the hollow pipe. Considering the geometric scaling law Cl 1/10, it can be obtained that the freezing target of this test is to make the frozen wall thickness reach 260 mm, which means that the temperature of the red points in Figure 5 has reached the freezing point (−0.5°C)
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